(C) 1996, The Institute For Ordinary Research, Director: Rob Peterson
The Institute has been asked to confirm the popular adage that the average person is thermodynamically equivalent to a 100 watt light bulb. The argument below indicates that the only thing missing is the light and the glass package.
100 Watts = 100 joules/second by definition. Mr. Joule studied the heat produced while boring cannon barrels and the international unit of heat received his name. Continuing, the energy produced or used per day if this power level is sustained for 24 hours is:
100 watts = 100 j/s (x 86400 sec/day) = 8,640,000 j/day
A joule of energy is equivalent to 0.2388 calories. This is the standard thermodynamic calorie roughly defined as the energy required to raise the temperature of one gram of water by one degree centigrade. So,
100 watts = 8,640,000 j/day = 2,063,233 calories/day.
This is probably the most confusing part: a "food calorie" is actually 1000 standard calories from thermodynamics, or a kilocalorie. Apparently, the food guys have simply shortened the proper kilocalorie to "calorie". Let's call the kilocalorie the Kalorie to keep them separate. Anyhow, you see that now we have
100 watts = 2,063 Kalories/day
which is close to the food intake rate of the average adult. This adult must radiate this same power (on average), else he/she would increase in temperature without limit. During heavy work periods, the kaloric use rate does increase above this to perhaps double or triple the average. A fairly small percentage of this goes to the actual work being done but most of this is expended inside the body because it is a very inefficient machine. So, If one spends an hour at 200w, the surface temperature rises (allowing more heat to radiate from the skin and be cooled convectively by air) and energy is stored in the heat capacity of the body (mostly water) to be dissipated by the body over the next hour or so during rest.